1. Field of the Invention
The present invention relates to a method for repairing a photomask by gallium chelation, and particularly to a method for repairing a phase shift photomask by gallium chelation with water-soluble polymers.
2. Background Art
Repairing mask defects with a focused ion beam (FIB) of gallium often results in a gallium-implanted area due to scan. This implanted layer reduces the light transmission by up to about 40 percent, depending on the wavelength. The effect of localized transmission loss becomes printable at the extremes of the focus-energy matrix of wafer printing. The existing methods for gallium removal give rise to quartz damage, side-etching or critical dimension (CD) variation.
Focused ion beam tools are typically used for binary and MoSiON phase shifting mask (PSM) defect repair. FIB tools use a finely focused beam of gallium ions that can be operated at low beam currents for imaging, or high beam currents for site-specific sputtering of chrome and other materials. However, the gallium-induced imaging and milling processes have the side effect of gallium being implanted on quartz. The gallium-implanted area absorbs light of 248 nm and 193 nm, giving rise to a localized transmission loss, printable defects on a wafer at the process split corners. A current gallium removal method employs plasma or corrosive chemicals for full-field quartz etching on masks. This method has some disadvantages, such as quartz damage, side-etching, and large critical dimension (CD) variation between the imaging and the non-imaging area after post-treatment. Moreover, this corrosive base is not suitable for sensitive materials like MoSiON PSM, because it damages the PSM shifter and results in phase angle and transmittance loss.
The repair of photomasks by sputter removal of chrome and other materials with a focused ion beam of gallium results in an attenuated layer of gallium implanted on the scanning or sputtering area. As noted above, this implanted area reduces the light transmission through it by up to about 40 percent, depending on the wavelength of the illumination. The effect of localized transmission loss will “print out” on the process wafer after exposure and produce CD variation. Further to the above discussion, current methods employ gas assisted etching (GAE) or direct sputtering on the quartz area, but are confined to the repaired area, and not all of the scanning image area. The ion plasma and corrosive chemical treatments are used for full-field etching on the photomask for removing gallium stains. These methods have disadvantages, e.g., quartz damage, side-etching, and larger CD variation between imaging.
Focused ion beam machines are typically used to carry out mask repair for Cr binary and phase shift masks. As is known, the gallium-induced imaging process results in gallium stains on the quartz area. At present, strong basic chemicals are used to remove such stains. However, this corrosive method impacts CD variation and induces side-etching, resulting in e.g., peeling.
The present invention relates to a gallium-removing process using a water-soluble polymer containing polyligands as a “catcher” for the metal chelation process. The present invention further relates to an improved gallium-removing cleaning process. This process is based on the concept of gallium chelation, with designed water-soluble polymers attached to multiple ligands for chelation.